ASoC: sh: fsi: change fsi->mst_ctrl to master->a/b_mclk

There was a strange part where fsi->xxx had been used
for fsi_master_xxx function instead of master->xxx in current FSI.
This patch modify it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>